1. Field of the Invention
The present invention relates to a combustion control system for an internal combustion engine, and, more particularly, to a combustion control system for an internal combustion engine which operates selectively with or without the exhaust gas recirculation.
2. Description of the Prior Art
As a type of fuel injection control in the internal combustion engine it is known to control the amount of fuel injection based upon the intake pressure or, in fact, vacuum in the intake passage of the engine according to a prearranged correlation between the intake pressure and the requirement for fuel injection in the engine. When such a fuel injection control system is employed in the internal combustion engine which operates selectively with or without the exhaust gas recirculation, the amount of fuel injection determined from the prearranged correlation between the intake pressure and the amount of fuel injection needs to be modified according to whether the engine is operating with the exhaust gas recirculation or not with the exhaust gas recirculation, because on the one hand the intake pressure rises (or the intake vacuum decreases) by the introduction of exhaust gas into the intake passage in respect to the same required amount of fuel injection, while on the other hand the requirement for fuel injection should desirably be raised for the same amount of fresh intake air under the exhaust gas recirculation to make a compensation for a reduction of combustibility of fuel due to mixing of the exhaust gas into fresh air. Such an art of modification of fuel injection is described in, for example, Japanese Patent Laying-open Publication No. 48-27130 (1973) and Japanese Patent Laying-open Publication No. 61-4836 (1986).
It is also known to control on and off of the exhaust gas recirculation by an exhaust gas recirculation control valve powered by the intake vacuum of the engine. Such a control system is unavoidably bound with a delay of operation that the exhaust gas recirculation is virtually started or stopped after the lapse of a substantial period from the time point of decision by the control system for the starting or stopping of the exhaust gas recirculation. Therefore, it has already been proposed to incorporate a delay time control in the fuel supply control system for the internal combustion engine operable with the exhaust gas recirculation as described in, for example, Japanese Patent Laying-open Publication No. 59-192838 (1984), Japanese Patent Laying-open Publication No. 60-169641 (1985) and Japanese Patent Laying-open Publication No. 61-4836 (1986).
However, the amount of delay in response in the starting and stopping of the exhaust gas recirculation differs according to the intake pressure and the rotational speed of the engine. Therefore, if the amount of delay is set to a constant value, the amount of fuel supply is not always ensured to be proper according to a difference in operational conditions of the engine. For example, if the delay time at the starting of the exhaust gas recirculation is determined to an amount appropriate for low load operation of the engine, the amount of delay time would be deficient in high load operation of the engine, because the intake vacuum decreases in high load operation of the engine, thereby causing too early modification of the fuel supply amount for exhaust gas recirculation. Therefore, if the modification is a fuel decreasing modification, the engine must operate with too lean fuel air mixture, and the stability of the engine is deteriorated.
On the other hand, fluctuations of the delay in response in the starting and stopping of the exhaust gas recirculation also substantially affect the optimization of the ignition timing for starting the combustion of the fuel-air mixture in the combustion chamber of the engine, because the most desirable timing for ignition substantially differs according to the rate of exhaust gas recirculated into the combustion chamber. Therefore, it is desirable if the ignition system is adapted to the virtual delay of starting and stopping of exhaust gas recirculation.